CN116802473A

CN116802473A - Tube testing apparatus and method

Info

Publication number: CN116802473A
Application number: CN202180089090.4A
Authority: CN
Inventors: P·罗伯茨; R·斯图尔特; R·凯特尔
Original assignee: Weidege Pipeline Technology Co ltd
Current assignee: Weidege Pipeline Technology Co ltd
Priority date: 2020-11-10
Filing date: 2021-10-22
Publication date: 2023-09-22
Also published as: CA3198125A1; GB2592704A; JP2023547962A; EP4244598A1; US20240011878A1; GB202017701D0; WO2022101605A1; GB2592704B

Abstract

An apparatus for testing rings cut from a tube, comprising: a plurality of test chamber sections defining a test chamber for receiving a ring to be tested when placed together, the test chamber sections comprising at least a first test chamber section defining a first inner face of the test chamber and a second test chamber section defining an opposite second inner face of the test chamber, the first and second inner faces defining an annular space arranged to receive the ring to isolate an inner side from an outer side of the ring; means for clamping the test chamber sections together to form a chamber; a fluid inlet port in one of the chamber portions to allow pressurized fluid to enter the chamber outside of the ring when the ring is received in the chamber; and one or more sensors for measuring strain and deformation of the ring and fluid pressure, wherein the annular spaces are aligned with each other, wider than the wall thickness of the ring to be tested, and substantially completely flat.

Description

Tube testing apparatus and method

Technical Field

The present disclosure relates to an apparatus for testing pipes, such as for forming underwater pipelines, and to a method of using the apparatus for pipe testing.

Background

Ultra-deep water reservoirs of natural gas and/or oil are evolving throughout the world. Until recently ultra-deep water was defined as any depth greater than about 1000 meters. However, so many pipes are installed in a depth greater than this depth that the definition of ultra-deep water is now over 2000 meters.

The pipeline is typically installed empty, i.e. filled with air at ambient pressure, and only after installation is completed is filled with oil or gas under pressure. The main risk experienced during installation of these deep water pipes comes from the pressure exerted by the water, which causes the pipe to deform out of its original circular shape and into an almost flat configuration. This is known as external pressure collapse and if uncontrolled, may result in total loss of the pipe. Thus, the size (i.e., diameter and wall thickness) of ultra-deep water pipes, as well as the material properties, are potentially constrained by external pressure collapse.

This is quite different from the design of conventional shallow water or land pipes where the wall thickness is sized to resist internal pressure from the fluid to be transported rather than external pressure.

Various theoretical studies on external pressure collapse have been made, and numerical simulation has also been used to calculate the maximum water depth at which a pipe having a specific size can be safely installed. However, the consequences of external pressure collapse are so great that these theoretical studies are insufficient to confidently manage risk. Furthermore, the potentially most important method of reducing such localized collapse by increasing the wall thickness of the tube is so expensive and may not be technically feasible that the proposed pipe is likely not commercially viable. This in turn increases the likelihood of abandoned production of the natural gas or oil reservoir.

In addition to basing all designs on theoretical results, another option is to conduct additional tests. Indeed, historically, several tests have been performed on a range of tube wall thicknesses. These tests involve placing the complete tube length of a particular manufactured tube in a particular pressure chamber and increasing the external pressure until collapse occurs. Currently, the number of laboratories with appropriate facilities is very limited and the testing costs are very expensive.

Some specifications have been developed to provide a basis for calculating the dimensions of pipes that need to be run at a particular large depth. These specifications include safety factors that are intended to ensure that when manufacturing a 1000 km length pipe, natural variations in pipe dimensions and material properties do not disrupt the pipe's ability to withstand external pressures without collapsing. However, these factors are based on collapse testing of a previously few complete tube lengths. The possibility of performing such a test on the entire pipe length (also referred to in the industry as a "pipe joint") during the manufacturing process is not practical because the test takes a long time to set up and complete, which of course damages the pipe under test.

One pipe can submerge or fail the whole pipeline if only one pipe joint collapses. There is a direct analogy to the "weakest link in a chain" regarding pipe failure due to external pressure collapse. Whereas the practice specifications are based on collapse test results for a small limited number of pipe joints, the design specifications introduce factors to allow all possible variations of many parameters affecting the collapse pressure, thereby increasing the wall thickness of the entire deep water route.

Recently, improved testing methods have been developed that aim to replicate the effects of external pressure causing collapse of the pipe joint, and these methods are easy (and more cost effective than historical testing methods) to set up and complete.

These improved test methods are based on the following recognition: the deformation that causes the collapse of the external pressure is uniform along the tube, and therefore, for rings cut from the tube, the collapse of the external pressure occurs as compared to the entire length of the tube joint of the tube that is fully subjected to the external pressure.

A prior art tube testing device for implementing an improved testing method is known from WO 2008/114049.

Such pipe testing equipment has proven to be very effective for testing thick-walled pipes in ultra-deep water. However, problems arise when testing tubes that are subject to what is commonly referred to as elastic buckling collapse under extreme hydrostatic loads, such as tubes having a diameter (D) to wall thickness (T) ratio D/T >25, which can be used at lower depths, including depths between 150 and 1000 meters. In addition, a certain level of expertise and precision is required in implementing these test methods. These tests are typically performed by a skilled artisan in a tube testing laboratory.

Disclosure of Invention

The present application aims to provide an improved tube testing apparatus which allows for non-destructive testing of tubes subject to elastic buckling collapse under extreme hydrostatic loads, and also to provide an improved tube testing apparatus which allows for efficient non-destructive testing of tubes outside of a dedicated test laboratory, allows for accurate repeatable operation by less skilled individuals, and allows for higher throughput of test samples, irrespective of the wall thickness or other characteristics of the test tube to be tested.

Representative features are set forth in the following clauses which may be presented alone or in any combination with one or more features disclosed in the text and/or drawings of the specification.

According to a first aspect of the present application there is provided apparatus for testing rings cut from a tube, comprising: a plurality of test chamber sections defining a test chamber for receiving a ring to be tested, the test chamber sections comprising at least a first test chamber section and a second test chamber section, the first test chamber section defining a first inner face of the test chamber and the second test chamber section defining an opposite second inner face of the test chamber, the first inner face and the second inner face defining an annular space arranged to receive the ring to isolate an inner side from an outer side of the ring; means for clamping the test chamber sections together to form the chamber; a fluid inlet port in one of the chamber portions to allow pressurized fluid to enter a chamber external to the ring when received in the chamber; and one or more sensors for measuring strain and deformation of the ring and fluid pressure, wherein the annular spaces are aligned with each other, wider than the wall thickness of the ring to be tested, and substantially completely flat.

With such an arrangement, the ring is received between the first inner face and the second inner face, the pressurized fluid entering an outer annular space external to the ring, the outer annular space being defined by the test chamber portion and the ring when the ring is received in the chamber.

The device omits the sealing elements associated with the first and second inner faces required in the referenced prior art. The annular space omits all sealing elements. More precisely, the device is arranged for use with a test ring that is itself provided with sealing elements, or with a test ring that is devoid of sealing elements, so that an arrangement is provided that omits all sealing elements between the test ring and the device. The arrangement of the test ring provided with sealing elements is directly opposite to the prior art. Both of these arrangements uniquely allow testing of tubes subject to cross-sectional deformation prior to failure.

The ring is received by the first inner face and the second inner face because the ring is located between the first inner face and the second inner face during testing. For example, in an arrangement in which the ring is provided with a sealing element, the inner face may form a seal with the ring for isolating the inner side of the ring from the outer side. Otherwise, for example, in an arrangement omitting the sealing element, the inner face may clamp the ring to isolate the inner side of the ring from the outer side. In this case, the clamping force will not prevent the test ring from deforming/moving laterally during testing.

When the test ring is provided with a sealing element, the seal between the ring and the device and the test pressure are maintained by deformation of the ring. The first inner face and the second inner face form a seal with the ring. The annular space is completely flat and allows the required movement, the sealing element moving with the ring when the ring is deformed to maintain a seal against the completely flat annular space.

To minimize friction, the annular space is preferably coated with a lubricant. Preferably, the lubricant prevents metal-to-metal contact between the ring and the first and second inner faces of the apparatus. More preferably, the annular space is polished in order to minimize friction. They may be polished.

According to a further aspect of the present application there is provided an apparatus as described in detail above in combination with a ring to be tested, the ring comprising substantially parallel end faces, each end face being provided with a sealing element.

The end face is preferably slotted for receiving a sealing element. The sealing element is preferably resilient and may comprise an O-ring, a lip seal or other. It is to be noted that the form of the sealing member is not particularly limited. They may comprise any suitable resilient or self-powered sealing element.

According to a further aspect of the present application there is provided a method of testing a ring cut from a tube using the apparatus described above, the method comprising:

a. cutting the ring from the tube;

b. fitting an accessory to measure strain and deformation of the ring;

c. fitting the ring into the apparatus and mounting a sealing element on an end face of the ring; and

d. the apparatus was used to apply pressure and strain and deformation measurements were recorded.

The method may include the step of machining an end face of the ring to form a groove for receiving the sealing element.

The arrangement of providing the test ring with sealing elements is different from the convention. Because of concerns about influencing the test results, any modifications to the pipe ring are strictly avoided in the prior art, especially in the case of thick-walled pipes for ultra-deep water, which require large sealing elements due to the extremely high test pressures. That is, any modification to the ring results in a concern that the sample no longer represents the pipe joint from which it was cut, and in a concern about the reliability of the test. Furthermore, the polishing of the ring is much simpler than the polishing of the annular space on the device, as is now uniquely proposed. Furthermore, it must be noted that, based on the teachings of the prior art, the failure mode of thick-walled tubes tested with the prior art apparatus is different from that for thin-walled tubes, leaving no incentive for further consideration.

However, the present inventors have determined through extensive studies that the presently proposed arrangements against conventional ideas are not only viable, but in some cases present significant benefits, particularly in the testing of thin walled tubes. Studies have determined that the material loss can be small enough and that the effect on the relevant bending resistance is almost negligible because it is located on the central axis. For example, an O-ring groove may be implemented without compromising test integrity while maintaining a seal during cross-sectional deformation to make measurements that were previously considered impossible. The prior art will lose tightness when the cross section of the ring under test is deformed.

As described above, when the sealing element between the test ring and the device is omitted, the pressurized fluid will leak controllably. The apparatus is preferably provided with a sump for collecting fluid leaking through the ring from outside the ring. While the annular space may be completely flat, as discussed further below, the annular space may also be substantially completely flat due to the provision of one or more radial grooves or isolated spacer protrusions. The pressurized fluid preferably comprises petroleum or alternatively a viscous fluid.

It should be noted that the principles of the present application may be applied to the testing of tubes having a wide range of diameters and wall thicknesses, and that the present application is not limited to use with only the thin walled tubes cited herein.

Drawings

Non-limiting embodiments of the present application will now be discussed with reference to the following figures:

fig. 1 shows an exploded perspective view of a test device and a ring to be tested according to a first embodiment;

FIG. 2 shows a cross-sectional view of the test apparatus of FIG. 1 in an assembled state, with the test ring in place, ready for testing;

fig. 3 shows detail C of fig. 2; and

fig. 4 shows a cross-sectional view of a test apparatus according to a second embodiment in an assembled state, with the test ring in place ready for testing.

Detailed Description

Testing of the long sections of individual pipe joints has shown that the deformation resulting in external collapse is uniform along the pipe. This observation is supported by theoretical studies and numerical simulations. This means that the occurrence of external pressure collapse will be the same for a ring cut from a tube as for a complete joint length of a tube that is fully subjected to external pressure. The test method of the application is therefore based on cutting short sections from the tube and machining the ring to a uniform length. The ring is placed in a rigid frame that allows the machined face of the ring to be sealed so that pressure can be applied only to the outer circular surface of the ring. The inner circular surface of the ring is kept at ambient pressure and is therefore suitable for attaching means to measure strain and deformation caused by pressure on the outer circular surface of the ring.

By sealing with the end face of the ring (or by controlled fluid leakage), the pressure can be limited to only on the outer circular surface of the ring. When a seal is applied or when a seal is omitted, as discussed further below, the arrangement is configured such that the ring is not subjected to substantial forces parallel to the end faces, such that deformation of the circular face of the ring is prevented.

Pressure is applied from an external pump such that the pressure is increased or decreased by adding or subtracting a specified volume of fluid from the space surrounding the outer circumferential surface of the ring. This arrangement allows the radial deformation of the ring caused by the pressure on the outer cylindrical surface to increase or decrease in a controlled manner.

A typical test will include the following steps:

a. cutting the ring from the tube;

b. fitting an accessory to measure strain and deformation of the ring;

c. fitting the ring into the device (whether there are sealing elements on the end face of the ring-depending on the arrangement); and

It may also be useful to plot the applied pressure against the measured maximum strain to detect the onset of an accelerated non-linear decrease in ring diameter with increasing pressure, independent of any leakage of hydraulic fluid past the seal.

Fig. 1 shows an exploded perspective view of an apparatus according to a first embodiment of the present application and a ring cut from a tube forming a test piece. Figures 2 and 3 show a device with a test ring mounted. It should be noted that fig. 2 and 3 are schematic for ease of illustration and do not show the actual tolerances between the depicted elements.

As shown, the apparatus comprises a plurality of test chamber sections 2, 3, 4 which, when placed together (as best shown in fig. 2), define a test chamber for receiving a ring 6 to be tested. The test chamber sections 2, 3, 4 comprise at least a first test chamber section 2 and a second test chamber section 4, the first test chamber section 2 defining a first inner face 7 of the test chamber and the second test chamber section 4 defining an opposite second inner face 8 of the test chamber. The first inner face 7 and the second inner face 8 define an annular space arranged to form a seal with the ring to isolate the inner side from the outer side of the ring. The apparatus comprises means 9, 10 for clamping together the test chamber parts to form a chamber. While the clamping means may take any suitable form, in the exemplary arrangement of the application they comprise bolts 9, the bolts 9 being received by suitable openings 11 and nuts 10 through the test chamber portions 2, 3, 4, as will be readily appreciated by those skilled in the art. The clamping means is not limited thereto. A fluid inlet port (not shown) is provided in one of the chamber portions to allow pressurized fluid to enter the chamber outside the ring when contained therein. The present apparatus comprises one or more sensors (not shown) for measuring the strain and deformation of the ring 6.

The annular spaces are aligned with each other. They are wider than the wall thickness of the ring to be tested. They are uniquely completely flat. The annular space itself is free of any seals. They define a continuous flat surface.

The annular space may be configured according to the pipe to be tested and its diameter and width set accordingly. For example, the annular space has an inner diameter 5 to 10 cm smaller than the inner diameter of the ring to be tested and an outer diameter 5 to 10 cm larger than the outer diameter of the ring to be tested.

The annular space is preferably polished and coated with a lubricant. The annular space may be polished.

The ring to be tested comprises substantially parallel end faces, each provided with a sealing element 12. The sealing element 12 in the arrangement of the application comprises an O-ring. In alternative arrangements they may comprise lip seals or any alternative form of pressure activated belt pressure seals.

The first inner face 7 and the second inner face 8 of the apparatus are joined by the sealing element 12 of the test ring 6 to form an outer annular space 14, the supply of pressurized hydraulic test fluid being accessible to the outer annular space 14 through a suitable inlet port (not shown). As shown, the central void 13 inside the test ring 6 is preferably open to the atmosphere, wherein a convenient passage is provided for any instrument/cable for connecting strain gauges (not shown) on the inner cylindrical surface of the test ring 6.

The force holding the test chamber sections 2, 3, 4 together (and thus the force applied to the test ring/sealing element by the first and second inner faces/annular spaces) is sufficient to cause the outer annular space 14 to seal the sealing element 12 against both internal and external pressures. As will be readily appreciated by those skilled in the art and discussed further below, the tolerances are selected such that leakage from the outer annular space 14 to the central void 13 does not occur while avoiding excessive limiting friction on radial movement of the ring 6 radially inward under hydraulic load.

The movement of the test ring during cross-sectional deformation during testing will now be further considered for tolerance and lubrication.

The device is configured to uniquely maintain a leak-proof seal between the test ring and the test device while allowing the cross-section of the test ring to deform. In order to make the test as accurate as possible, it is desirable to minimize metal-to-metal contact between the test sample and the test equipment and limit friction between the sealing element and the surface of the test equipment.

As will be appreciated by those skilled in the art, to limit friction, it is desirable that: avoiding excessive static compression of the sealing element, as this increases the contact pressure and thus the friction; avoiding excessive clearances, as this may allow the sealing element to squeeze into the clearance under pressure and cause a "wedging" effect; providing effective lubrication; and to implement appropriate tolerances.

The following tolerances are relevant:

1. the first and second inner faces 7, 8 of the apparatus are of a tolerance for parallelism above and below the test ring.

2. The flatness tolerances of the upper and lower faces of the test ring are measured by the first and second inner faces 7, 8 of the apparatus.

3. Surface finish tolerances of the annular space.

The friction-limiting measures are selected accordingly, taking into account the ring to be tested and the test parameters to be carried out.

In an exemplary, non-limiting arrangement, for a tube having a diameter of 42 inches (1.07 meters) and configured for a depth between 200 and 500 meters, the following configuration may be employed:

the gap between the test ring and each of the first inner face 7/second inner face 8 is 1.5 milli-inches (0.038 mm) to 4 milli-inches (0.127 mm).

The initial compression of each sealing element (O-ring) is 10% to 20%.

Flatness and parallelism tolerance

As described in detail above, during movement of the test ring during testing, configured for maintaining clearance and sealing element compression tolerances.

Fig. 4 shows a view of a device according to a second embodiment, provided with a ring in place for testing. It should be noted that fig. 4 is also schematic, and true tolerances between the depicted elements are not shown for ease of illustration.

According to a second embodiment, the sealing element 12 is omitted from the test ring. In this embodiment, the apparatus is substantially the same as that discussed with reference to the first embodiment, as depicted in fig. 1-3, but relies on controlled leakage of pressurized fluid rather than sealing. The fluid in the annular space 14 will be replenished at a suitable rate and depending on the leakage being controlled to control the increase in pressure. Common features between this embodiment and the first embodiment will not be discussed in detail, wherein the discussion of any such features above is directly applicable to this embodiment, as will be appreciated by those of skill in the art.

The apparatus adds a sump 15 for collecting fluid leaking from the annular space 14 past the test ring 6 and into the region of the central void 13. Pressurized fluid escaping through the nominal gaps at the top and bottom of the sample ring is discharged into the sump through one or more discharge holes 16, as indicated by the arrows in fig. 4.

Preferably, the fluid is recycled by the apparatus as shown in the arrangement of the present application. For this purpose, a low-pressure pump 17, a regulating tank 18 configured to degas and filter the fluid may be provided. The conditioning tank preferably feeds a supply tank 19, and the supply tank 19 may be integrally formed with the conditioning tank as shown, or may be provided separately. The high pressure pump 20 injects pressurized fluid into the annular space through the pressurized line 22 in a controlled manner to maintain the sample at an increased pressure.

It must be understood that in alternative arrangements, the fluid need not be recirculated and the apparatus may be modified accordingly.

By properly setting the tolerances, as set forth above in 1, 2, and 3, leakage of pressurized fluid may be controlled.

As with the first embodiment, the annular space may be entirely flat. In other embodiments, the annular space may be provided with one or more radial grooves and/or one or more protrusions, as described above. The grooves/protrusions are preferably isolated/spaced from each other.

In the arrangement of fig. 4, a plurality of projections 21 are provided. These protrusions 21 preferably comprise three or more small sharp or sharp edged platforms to support the sample at a preferred tolerance away from the bottom annular space. As will be appreciated by those skilled in the art, they are configured so as not to affect the test results to any significant extent. The provision of the platform ensures that the pressurized fluid flows over the upper and lower surfaces of the sample ring.

As in the case of the first embodiment, the movement of the test ring does not affect the test compared to the prior art arrangement.

Referring to fig. 5, another alternative arrangement is shown that can be applied to any of the arrangements described above, whether with or without a seal. This means that reservoir 32 is optionally introduced into the pressurized system to allow for variation in the "hydraulic stiffness" of the pressurized system.

As will be appreciated, fig. 5 shows a schematic arrangement for illustration purposes only. Which shows the introduction of a reservoir 32 into the arrangement of figure 4 (shown with the option of a separate regulating tank 18 and supply tank 19). However, as noted above, the reservoir 32 may be incorporated into any of the arrangements described above, including the arrangements discussed with reference to fig. 1-4.

The introduction of the reservoir 32 provides a means to vary the stiffness of the pressurized system to enhance the visibility of the "permanent set limit", i.e., when the non-recoverable plastic strain caused by a standard increase in pressure exceeds a predefined acceptable level. This is valuable in the following cases: such permanent deformation of the tube cross section is a selected practical acceptance threshold beyond which the level of permanent deformation of the tube cross section is considered unacceptable for practical reasons even though the tube integrity has not been compromised.

As will become apparent from the discussion below, the form of the reservoir 32 is not particularly limited. For example, any conventional gas-supported reservoir may be implemented, as will be readily appreciated by those skilled in the art.

Referring to the arrangement of fig. 5, when the valve 30 is closed, the system has a constant maximum stiffness and the pressure increase is relieved by very little strain. Opening valve 30 and filling reservoir 32 with compressed gas (such as, but not limited to, any of dry air, nitrogen, or carbon dioxide) provides more system flexibility by opening valve 31 to, for example, a first level (indicated by dashed line 33), wherein a standard pressure increase would require more strain to release. Further increases in gas pressure will drive the fluid down to, for example, a second level (indicated by dashed line 34), where a larger gas volume provides even greater flexibility whereby a standard system pressure rise, matching the gas pressure rise maintaining the second level, will require even more strain of the sample loop to be relieved. This means that the sensitivity with which an operator can detect the "permanent set limit" described below can be effectively enhanced, allowing for a faster and easily managed non-destructive testing process.

As will be appreciated by those skilled in the art, the reservoir may take any suitable known form.

The method and apparatus according to the application show a number of advantages over the prior art. They allow testing representative samples of the test rings taken from all the pipe joints required for long pipes to give direct physical quantification evidence of the ability of each of these samples to resist external hydrostatic collapse. The collapse tolerance of each sample test ring can be confidently maintained as representing the collapse tolerance of the joint from which it was cut. Use of the present application in the manner described may allow for a reduction in factors currently used in the design process to increase the wall thickness of the overall pipeline. The joint from which each test ring is cut can still be used as a production joint and is not wasted. The end result may be a highly significant reduction in the wall thickness of the conduit, which will provide improved commercial availability of the conduit and significant cost savings. They uniquely allow testing of pipes that are typically subject to elastic buckling collapse under extreme hydrostatic loads, as compared to the cited prior art. They further provide accurate repeatable operation by less skilled individuals and allow for higher throughput of test samples. This allows testing of many samples to be performed at the source, in a tube mill as part of the production process, or otherwise. The disclosed apparatus also allows multiple tests to be performed without any components being changed.

Many alternative arrangements and modifications of the apparatus as described herein will be readily appreciated by those skilled in the art within the scope of the appended claims.

The terms "comprises" and "comprising," when used in this specification and claims, are inclusive and mean that the stated features, steps, or integers are included. These terms should not be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the application in diverse forms thereof.

While certain exemplary embodiments of the present application have been described, the scope of the appended claims is not intended to be limited to only these embodiments. The claims are to be interpreted literally, purposefully, and/or include equivalents.

Claims

1. An apparatus for testing rings cut from a tube, comprising:

a plurality of test chamber sections defining a test chamber for receiving a ring to be tested when placed together, the plurality of test chamber sections including at least a first test chamber section and a second test chamber section, the first test chamber section defining a first inner face of the test chamber and the second test chamber section defining an opposite second inner face of the test chamber, the first inner face and the second inner face defining an annular space arranged to receive the ring to isolate an inner side from an outer side of the ring;

means for clamping the plurality of test chamber sections together to form a chamber;

a fluid inlet port in one of the chamber portions to allow pressurized fluid to enter a chamber external to the ring when received in the chamber; and

one or more sensors for measuring strain and deformation of the ring and fluid pressure;

wherein the annular spaces are aligned with each other, wider than the wall thickness of the ring to be tested, and substantially completely flat.

2. The apparatus of claim 1, wherein the annular space is coated with a lubricant.

3. The apparatus of claim 1 or 2, wherein the annular space is polished.

4. An apparatus as claimed in any one of the preceding claims, wherein the annular space is polished.

5. The apparatus of any one of the preceding claims, comprising a pressurization system for providing the pressurized fluid to the fluid inlet port, wherein the pressurization system comprises a reservoir.

6. The apparatus of claim 5, wherein the reservoir comprises a gas-bearing reservoir.

7. The apparatus of claim 5 or 6, wherein the reservoir is configured to change a stiffness of the pressurization system.

8. Apparatus according to any one of the preceding claims in combination with a ring to be tested, the ring comprising substantially parallel end faces, each end face being provided with a sealing element.

9. The combination of claim 8, wherein the end face is slotted for receiving the sealing element.

10. A combination as claimed in claim 8 or 9, wherein the sealing element is resilient.

11. The combination of any of claims 8 to 10, wherein the sealing element comprises an O-ring or a lip seal.

12. The combination of any one of claims 8 to 11, wherein D/T >25, wherein D is the outer diameter of the ring and T is the wall thickness of the ring.

13. A combination according to any one of claims 8 to 12, wherein the inner diameter of each annular space is 5 to 10 cm smaller than the inner diameter of the ring to be tested and the outer diameter of each annular space is 5 to 10 cm larger than the outer diameter of the ring to be tested.

14. A method of testing a loop cut from a tube using the apparatus of any one of claims 1 to 7, the method comprising:

a. cutting the ring from the tube;

b. fitting an accessory to measure strain and deformation of the ring;

15. The method of claim 11, further comprising machining an end face of the ring to include a groove for receiving the sealing element.

16. The apparatus of any one of claims 1 to 7, further comprising a sump for collecting any pressurized fluid leaking from outside into the interior of the ring.

17. The apparatus of claim 16, further comprising a pump for recirculating the pressurized fluid from the sump.

18. The apparatus of any one of claims 1 to 7, 16 or 17, wherein the annular space comprises one or more radial grooves and/or one or more protrusions.

19. The apparatus of claim 18, wherein the grooves/protrusions are spaced apart from each other around the annular space.

20. The apparatus of any one of claims 1 to 7 or 16 to 19, in combination with a ring to be tested, the ring comprising completely flat, substantially parallel end faces.

21. The combination of claim 20 wherein the end face omits all sealing elements.

22. A combination as claimed in claim 20 or 21, wherein the inner diameter of each annular space is 5 to 10 cm smaller than the inner diameter of the ring to be tested and the outer diameter of each annular space is 5 to 10 cm larger than the outer diameter of the ring to be tested.

23. A method of testing rings cut from a tube using the apparatus of any one of claims 1 to 7 or 16 to 20, the method comprising:

a. cutting the ring from the tube;

b. fitting an accessory to measure strain and deformation of the ring;

c. fitting the ring into the apparatus; and